1. Field of the Invention
This invention relates to a transfer medium separating device in an electrophotographic copying apparatus. More particularly, it relates to a transfer medium separating device in an electrophotographic copying apparatus for separating a transfer medium from a photosensitive drum to which the transfer medium is electrostatically attracted.
2. Description of the Prior Art
An electrophotographic copying apparatus generally comprises a rotatable photosensitive drum, and various means disposed around the photosensitive drum successively in the direction of rotation thereof, such as electrostatic latent image forming means, means for developing electrostatic latent images, means for transferring the developed images to transfer mediums, and means for cleaning the photosensitive drum. In such an electrophotographic copying apparatus, the transfer medium is urged into contact with the photosensitive drum as by a corona discharger or a transfer roller during image transfer, with so intense an electrostatic attraction that the transfer medium cannot readily be separated from the drum. This necessitates the provision of some means for separating the transfer medium from the photosensitive drum.
There has heretofore been devised a separating means utilizing vacuum suction or a method of injecting a stream of air along the surface of the drum against the leading edge of the transfer medium. In the former, the separating mechanism must be rotated or moved in synchronism with the transfer medium. The latter also requires indispensably a means for providing the synchronization with the transfer medium. In either case, the use of a compressor or a pump is further required and this increases the cost of the apparatus, and increases the noise level of the machine, and the possibility of developer on the drum and/or the transfer medium being scattered within the apparatus.
A construction has therefore been proposed which comprises a separating belt stretched from the upstream to the downstream of the transfer means, and a separating roller disposed just downstream of the transfer means so as to deflect the separating belt away from the photosensitive drum at an angle of contact within 180.degree. So that the transfer medium transported between the photosensitive drum and the transfer means may be directed to the separating roller by the separating belt without one side edge portion of the transfer medium being brought into contact with the photosensitive drum. An example of such construction is shown in FIGS. 1 and 2 of the accompanying drawings.
In FIGS. 1 and 2, there is seen a photosensitive drum 1 having its peripheral surface formed by a photosensitive layer 1.sub.1, an image transfer charger 2, a set of transfer medium feed rollers 3, and a transfer medium guide 4 along which a transfer medium P may be fed to the photosensitive drum 1.
From the upstream of the image transfer charger 2, an endless separating belt 5 is stretched in contact with the photosensitive drum 1. Immediately downstream of the image transfer charger 2, there is provided a separating roller 6 rotatable at a velocity equal to or higher than the peripheral velocity V of the photosensitive drum 1, and the separator belt 5 is directed away from the photosensitive drum 1 at an angle of contact .alpha. (0.degree. to 180.degree.) so that the transfer medium P may be guided with a widthwise side edge portion thereof maintained out of contact with the photosensitive drum 1 from a position before the image transfer process; and, after having passed the image transfer charger, the transfer medium may be guided away from the photosensitive drum 1 with a predetermined tension, whereby the transfer medium may be separated from the photosensitive drum 1.
The separator belt 5 is located at a position corresponding to a width of about 5 to about 20 mm at one side edge of the transfer medium P.
Rollers 7 and 8 are for rotatively driving the separating belt 5 and a guide roller 9, connected to a spring 10 by hook means, serves to normally impart tension to the separating belt 5.
The separating roller 6 has its surface coated with rubber so as to ensure that the separating belt 5 can be moved therearound without slipping, and also to ensure that the transfer medium P interposed between the roller 6 and the separating belt 5 can be positively transported.
Designated by 11 is a belt for transporting the transfer medium P to a fixing station. A guide 12 is provided between the separator 6 and the belt 11. Numeral 13 denotes a blower for blowing air on the transfer medium P.
The transfer medium P, when fed to the photosensitive drum 1 via the feed rollers 3, 3 and through the guide 4, is brought into intimate contact with the photosensitive drum 1 by the image transfer charger 2 for a toner image to be transferred from the drum to the transfer medium. When this occurs, one side edge portion of the transfer medium P is forcibly retained out of engagement with the photosensitive drum 1 by the separating belt 5 intervening therebetween, and the transfer medium is transported along the underside of the belt 5 to the separating roller 6. The transfer medium is then directed along the separating roller 6 away from the photosensitive drum 1 at an angle of contact .alpha., so that the side edge portion of the transfer medium P is drawn apart from the photosensitive drum 1 as soon as it passes the image transfer position, and it is further separated from the photosensitive drum 1 as it advances. Thus, the separating action progressively propagates from the portion of the transfer medium adjacent to the separating belt toward the successive inner portions of the transfer medium, whereby the transfer medium is naturally and yet positively separated from the photosensitive drum 1.
After being so separated from the photosensitive drum 1, the transfer medium P droops onto the separating roller 6 and the friction between the transfer medium and the roller 6 acts to transport such transfer medium to the belt 11 via the guide 12.
In order to enhance such separating action, air is blown from the blower 13 against the transfer medium separated from the photosensitive drum 1. The air should preferably be blown in a direction along the wedge-shaped space between the photosensitive drum 1 and the transfer medium P separated therefrom. If the shown direction of air flow is adopted, the air will naturally be sent along the upper surface of the transfer medium into the above-mentioned space to thereby promote the separation and also urge the transfer medium P against the guide 12 and the belt 11 to prevent it from floating up, thus ensuring intimate contact of the transfer medium with the belt 11 which results in positive transport thereof.
As will be seen from this example, the separation system using a separating belt has very excellent features as compared with the other methods already described. More particularly, it eliminates any special drive source for the separation, and accordingly the need to use a synchronizing means for providing synchronization. Moreover, the separating action constantly occurs in the separating station to ensure positive separation of transfer medium from the drum.
The operation of the separating belt 5 may be divided into two actions, namely, the action of drawing a leading edge corner P.sub.1 of the transfer medium P from the photosensitive medium 1 at a point A (FIG. 3) to thereby provide a moment of separation, and the action of cooperating with the separating roller 6 to nip, draw up and transport the drawn-apart end of the transfer medium between points B and C to thereby propagate said moment throughout the entire width of the transfer medium.
Assuming that the surface of the photosensitive drum 1 and the surface of the separating roller 6 are spaced apart from each other by a distance d, the separating belt 5 is stretched over a common tangent AB (distance l) with the photosensitive drum 1 and the separating roller 6.
On the other hand, the transfer medium P is electrostatically attracted to and moved with the surface of the photosensitive drum 1. Even if the leading end edge of the transfer medium has passed the point A and one side corner P.sub.1 thereof has been drawn apart from the photosensitive drum 1 by the separating belt 5, the remainder of the transfer medium tries to move forward along the surface of the drum 1 while being attracted thereto. Thus, the corner P.sub.1 of the transfer medium is pulled on in the direction of arrow f by the remainder of the transfer medium which remains attracted to the drum 1, as illustrated in FIG. 4. Between the points A and B, however, the transfer medium P simply bears against the separating belt 5 and is not restrained from the movement in the direction perpendicular to the direction of travel. Consequently, the corner P.sub.1 cannot resist the pull force f, with a result that some deviation of the corner P.sub.1 arises in the direction perpendicular to the direction of travel. Such deviation increases with the distance from the point A and if the corner P.sub.1 is disengaged from the separating belt 5, separation of the transfer medium will no longer be possible. Therefore, before it is disengaged from the separating belt, the corner P.sub.1 must reach the point B whereat it can resist the pull force f. The increasing deviation may also hamper the subsequent separating action and cause oblique movement of the transfer medium P, and such increase in deviation must be avoided.
This in turn requires the distance l between A and B to be minimized, but it is not allowable to make such distance zero, that is, to bring the photosensitive drum 1 and the separating roller 6 into contact, because the contact therebetween would cause the developer on the photosensitive drum 1 to adhere to the separating roller 6 and contaminate the back side of the transfer medium P. Therefore, minimization of d and l to such an extent that no adhesion of the developer to the separating roller occurs is requisite for good separation by the system now under discussion.
Subsequently, the corner P.sub.1 of the transfer medium P passes the point B and is nipped between the separating belt 5 and the separating roller 6, whereby it is subjected to an upward draw. This upward draw occurring between B and C is more abrupt than between A and B, so that the pull force f becomes stronger. By causing the upward draw to continue against the pull force f, the electrostatic attraction between the transfer medium P and the photosensitive drum 1 may be overcome to permit the separating action to propagate throughout the width of the transfer medium P.
It is the friction force between the separating roller 6 and the transfer medium P that restrains, against the pull force f, the corner P.sub.1 of the transfer medium P from movement in the direction perpendicular to the direction of travel. The separating belt 5, however, is usually formed by a film of PETP or like material which is low in friction coefficient, and a high friction force between it and the transfer medium P cannot be expected. What creates a friction force between the transfer medium P and the separating roller 6 is the force with which the separating belt 5 urges the transfer medium against the separating roller 6, namely, the tension of the separating belt and the angle of contact .alpha. between the separating belt and the separating roller. In order to provide a friction force which will be sufficient to resist the pull force f, it is therefore necessary to cover the surface of the separating roller 6 with rubber of high friction coefficient, to provide a great angle of contact .alpha. and to provide a great tension of the separating belt 5. Nevertheless, the angle of contact .alpha. is limited by the construction of the copying machine and cannot be greater than a certain degree. Also, the separating belt 5 which is formed by a thin film of high molecular material should not have imparted thereto a tension exceeding a certain limit, otherwise the useful life of the belt will be decreased. Because of these practical limitations, the conventional separation system utilizing a separating belt has necessitated delicate conditioning for the separation effected only by the separating belt, and often involved the use of auxiliary means like the aforementioned air flow which is blown against the transfer medium at the separating station. This has unavoidably caused other problems in that some of the developer is scattered within the machine. In addition, the tension imparted to the separating belt is set at the allowable maximum and this is not sufficiently satisfactory in respect of the service life of the belt.